2022年, 第19卷, 第1期　
刊出日期：2022-01-28

  

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Research & Development
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  Structures and macrosegregation of a 2024 aluminum alloy fabricated by direct chill casting with double cooling field

  Cheng Zhu, Zhi-hao Zhao, Qing-feng Zhu, Gao-song Wang, Yu-bo Zuo, Gao-wu Qin

  《中国铸造》英文版. 2022, 19(1): 1-8. https://doi.org/10.1007/s41230-022-1030-5

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  Central region coarse grains and centerline segregation are common defects in aluminum ingots fabricated by direct chill (DC) casting. A double cooling field was introduced into the DC casting process to reduce these defects, whereby the external cooling was supplied by the mold and water jets, and intercooling was achieved by inserting a rod of the same alloy into the molten pool along the central axis of the ingot. Rather than forming a good metallurgical interface during solid-liquid compound casting, in the present work, the purpose of inserting the rod is to enforce internal cooling and consequently decrease the sump depth. Moreover, the insertion provides more nucleation sites with the unmolten á-Al particles. The structure and the macrosegregation of 2024 Al alloy ingots prepared by DC casting with and without the inserts were investigated. Results show that when the inserting position is 50 mm above the upper edge of the graphite ring, significant grain refinement in the central region of the ingot and a reduced centerline segregation are achieved.
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  Effects of Zr additions on structure and tensile properties of an Al-4.5Cu-0.3Mg-0.05Ti (wt.%) alloy

  Hamed Kamali, Erfan Kamali, Massoud Emamy

  《中国铸造》英文版. 2022, 19(1): 9-16. https://doi.org/10.1007/s41230-022-1094-2

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  The effects of different Zr additions (0.05wt.%-0.5wt.%) on the structure and tensile properties of an Al-4.5Cu-0.3Mg-0.05Ti (wt.%) alloy solidified under a high cooling rate (18 ℃·s^-1), in as-cast and T6 heat-treated conditions were studied. The as-cast structure of the alloy consists of equiaxed grains of α-Al with an average size of 64 μm which is unaffected by the Zr additions, indicating the ineffectiveness of Zr in the grain refinement of the alloy. Scanning electron microscopy, along with X-ray diffraction analysis revealed the presence of elongated θ-Al₂Cu at the grain boundaries; in addition, coarse Al₃Zr particles exist in the intergranular regions of the 0.5wt.% Zr-containing alloy. After the T6 heat treatment, the elongated θ particles were fragmented; however, the coarse Al₃Zr particles remained unchanged in the microstructure. Also, the formation of fine β'-Al₃Zr and θ″-Al₃Cu/θ'-Al₂Cu phases during T6 heat treatment was revealed by transmission electron microscopy. The results of the tensile tests showed that the Zr additions increase the strength of the alloy in both as-cast and T6 heat-treated conditions, but reduce its elongation, especially with 0.5wt.% Zr addition. The 0.3wt.% Zr-added alloy in the T6 heat-treated condition has the highest quality index value (249 MPa). Fractography of the fracture surfaces of the alloys revealed ductile fracture mode including dimples and cracked intermetallic phases in both conditions.
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  Effect of Zr on solidification and microstructure of a Ni-based superalloy with high Al and Ti contents

  Guang-di Zhao, Xi-min Zang, Fang Liu, Wei-wei Zhang, Shuo Gao, Xue Li, Jing Guo

  《中国铸造》英文版. 2022, 19(1): 17-26. https://doi.org/10.1007/s41230-022-1058-6

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  The total content of Al and Ti in advanced Ni-based wrought superalloys is up to 7.5wt.%, which makes it easier to form harmful nonequilibrium eutectic (γ+γ') and η phase. It has been reported that the addition of certain amount of Zr can modify precipitation of the nonequilibrium phases obviously, but the mechanism is still controversial. The effect of Zr ranging from <0.0006wt.% to 0.150wt.% on solidification behavior, segregation and microstructure of a Ni-based superalloy with high Al and Ti contents was investigated, eliminating the interferences of C and B. Results show that increase in Zr content significantly promotes the formation of eutectic (γ+γ'), η and Zr-rich phase in the interdendritic region. Besides the Zr-rich phase, Zr dissolves slightly in the eutectic γ' and obviously in the η phase. An interesting phenomenon is discovered that the Zr addition significantly increases the area fraction of liquid pools and enlarges the forming range of γ dendrites, which suggests that Zr markedly retards the solidification. Zr affects the eutectic (γ+γ') and η formation mainly due to the retard of solidification and dissolution of Zr in them. The retard of solidification obviously increases the residual liquid fraction and undercooling. Zr can serve as a forming element for the eutectic (γ+γ') and η phase, and the obvious dissolution of Zr in η phase significantly decreases the critical concentration of Ti for its precipitation.
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  Evolution mechanism of recrystallization in a nickel-based single crystal superalloy under various cooling rates during heat treatment

  Yong-shun Li, Wei-dong Xuan, Jian Yang, Yu-hao Zhou, Ge Song, Xing-fu Ren, Zhong-ming Ren

  《中国铸造》英文版. 2022, 19(1): 27-34. https://doi.org/10.1007/s41230-022-1017-2

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  The recrystallization behaviors of a nickel-based single crystal superalloy during heat treatment at 1,200 ℃ for 4 h with various cooling rates were studied. Results show that the thickness of recrystallization layer decreases with the increase of cooling rate. In addition, the microstructures of γ' phase in the recrystallization region are different in various cooling rates. In the high cooling rates (70, 100 ℃·min^-1), small size and high volume fraction of γ' phases are formed in the recrystallization region. It is also found that irregular fine secondary γ' phases are precipitated between matrix channels with an average size of 150 nm in the original matric (100 ℃·min^-1). The sizes of the secondary γ' phase decrease with the increase of cooling rate. In contrast, large size and small volume fraction of γ' phases are formed in the recrystallization region, and a grain boundary layer is formed under a low cooling rate (10 ℃·min^-1). The evolution mechanism of recrystallization at various cooling rates during heat treatment is analyzed.
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  Evolution mechanism of crystallographic orientation in grain continuator bars of a Ni-based single-crystal superalloy prepared by Bridgman technology during directional solidification

  Jiu-han Xiao, Wei-guo Jiang, Dong-yu Han, Kai-wen Li, Guo-jun Tong, Yu-zhang Lu, Lang-hong Lou

  《中国铸造》英文版. 2022, 19(1): 35-45. https://doi.org/10.1007/s41230-022-1139-6

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  Single-crystal rods with different diameters and deviation angles with respect to the solidification direction were produced by Bridgman rapid solidification method at withdrawal rates of 3 and 6 mm·min^-1 and used as grain continuators. The crystallographic orientation of the rods, which cross-sections were perpendicular to the solidification direction at different solidification heights, was measured by electron backscattered diffraction, while the corresponding microstructures were observed by optical microscopy. The mushy zone morphology and the distribution of the temperature gradient were simulated by the finite element analysis software ProCAST. The experimental results indicate that the crystallographic orientation of the single-crystal rods corresponds to the statistical average value of all the dendrite orientations in cross-section. The crystallographic orientation of the primary and secondary dendrites of each single-crystal rod at different cross-sections fluctuates irregularly within a small range (less than 4°). The crystallographic orientation of the dendrite in each single-crystal rod is not exactly consistent with each other and is affected by their branching mode of dendrites in the solidification space. In addition, the simulation results show that the mushy zone shapes and the temperature gradient of single-crystal rods change with the increase of solidification height during the solidification process. Finally, the evolution mechanism of the crystallographic orientations and the corresponding influence factors were analyzed and discussed.
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  Effect of electromagnetic stirring on metallurgical quality of GCr15 electroslag ingot

  Xiao-fang Shi, Tao Xu, Li-zhong Chang

  《中国铸造》英文版. 2022, 19(1): 46-54. https://doi.org/10.1007/s41230-022-1107-1

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  To further improve the metallurgical quality of electroslag remelting, remove the large inclusions in electroslag ingot and refine the solidification structure of electroslag ingot, an electroslag remelting furnace with electromagnetic stirring was designed, and the influence of different magnetic induction intensities on metallurgical quality of GCr15 electroslag ingot was studied. Inclusions with different sizes and types were analyzed by an ASPEX scanning electron microscope, and the morphology and composition of inclusions were further observed by a JSM-6510LV scanning electron microscope. The distribution of alloying elements on the cross section of electroslag ingot was analyzed by original position analysis (OPA). The results show that whether or not electromagnetic stirring is used, the inclusions in electroslag ingot are mainly composed of Al₂O₃, MnS, MnS-oxide and other oxides, among which Al₂O₃ is the most. Compared with an electroslag ingot without electromagnetic stirring, the number of inclusions decreases considerably, and the proportion of small inclusions increases while the proportion of large inclusions decreases when the electromagnetic stirring with remelting current of 1.1 kA and magnetic induction intensity of 62 Gs is applied. However, excessive electromagnetic force will cause the number and diameter of inclusions to increase again. Electromagnetic stirring has different effects on the segregation of different elements. Under the experimental conditions, the weak electromagnetic force with 1.1 kA and magnetic induction intensity of 108 Gs has little effect on the segregation of C, but decreases the segregation of P, and the excessive electromagnetic force aggravates the segregation of alloy elements.
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  Finite element simulation of real cavity closure in cast Ti6Al4V alloy during hot isostatic pressing

  Qian Xu, Wen Li, Ya-jun Yin, Jian-xin Zhou, Hai Nan

  《中国铸造》英文版. 2022, 19(1): 55-62. https://doi.org/10.1007/s41230-022-1173-4

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  The healing behavior of shrinkage cavity inside the cast Ti6Al4V alloy during hot isostatic pressing (HIP) was investigated experimentally by interrupted hot isostatic pressing tests. The X-ray micro computed tomography was used to record the morphology changes before and after hot isostatic pressing. The two-dimensional geometry obtained by the microCT scan was used in simulation to study the evolution of the real shrinkage cavity during hot isostatic pressing. Shrinkage cavities, shrinkage porosity and small gas pores can be effectively eliminated under proper HIP conditions. The two-dimensional morphology in the simulation results agrees well with the experimental results. This study reveals that plastic deformation, creep and diffusion are the main mechanisms of cavity closure during hot isostatic pressing. In addition, the simplified elliptical pores with aspect ratios at different positions were used to replace the real pores to further study the factors affecting the position of dimples after HIP by simulation. It is found that the position of the dimples mainly depends on the aspect ratio of the elliptical pore and the distance between the pore surface and the external surface of the geometric model.
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  Solidification structure simulation and casting process optimization of GCr15 bloom alloy

  Jie Li, Hao-tian Wu, Yang Liu, Yan-hui Sun

  《中国铸造》英文版. 2022, 19(1): 63-74. https://doi.org/10.1007/s41230-022-0077-7

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  Based on the solidification heat transfer model and CAFÉ model, the solidification structure of GCr15 bloom alloy was studied. Using nail shooting and acid etching experiments, the solidification models were verified. The secondary dendrite arm spacing (SDAS) model of GCr15 was obtained by simulation calculation and metallographic observation. With the increase of casting speed, the SDAS, equiaxed crystal ratio (ECR) and average grain size increase. With the rise of superheat, the SDAS increases in the 20–70 mm of thickness and decreases in the 80–160 mm of thickness. The ECR decreases and the average grain size increases with the increase of superheat. With the increase of specific water flow, both the SDAS and ECR decrease. The minimum average grain size is obtained when the specific water flow is 0.20 L·kg^-1. The central carbon segregation index is reduced from 1.11 to 1.075.
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  Oxidation induced ductility enhancement of Zr based metallic glass ribbons in vicinity of glass transition temperature

  Jin-tao Wang, Xue-feng Gao, Rui-chun Wang, Yin-xiao Wang, Fu-yu Dong, Wen-ying Xue, Hong-wang Yang

  《中国铸造》英文版. 2022, 19(1): 75-80. https://doi.org/10.1007/s41230-022-1102-6

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  Zr-based metallic glasses (MGs) possess a wide supercooled liquid region, which gives a wide processing window for superplastic forming to make microdevices with demanding size accuracy and surface finishing. The existence of oxygen may have an influence on the thermoplastic deformation process. Therefore, the effect of oxidation on the mechanical behavior of the MGs in the vicinity of glass transition temperature is of great significance for practical forming of MG components. In the present study, the effect of oxidation on tensile properties of Zr₅₀Cu₄₀Al₁₀ metallic glass was investigated. The tested samples were characterized by XRD and SEM analysis. For the samples tested in air, the strength decreases 187 MPa, 61 MPa and 59 MPa and the ductility increases 0.31, 0.36, and 0.77 at 420 ℃, 430 ℃, and 440 ℃, respectively, compared with those tested in flowing argon. ZrO₂ preferentially formed during the tensile testing at 420 ℃, and both ZrO₂ and Al₂O₃ oxides formed at 430 ℃. The dilution of Zr elements in the remaining amorphous matrix caused by preferential oxidation on the surface layer attributes to the decrease in strength and enhancement in ductility of the Zr₅₀Cu₄₀Al₁₀ metallic glasses.
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  Regulative effect of Mg/Si ratio on microstructure evolution and mechanical properties of cast Al-Li-Mg-Si alloys

  Xue-jian Zhang, Hong-wei Wang, Feng-bai Ye, Chun-ming Zou, Zun-jie Wei

  《中国铸造》英文版. 2022, 19(1): 81-88. https://doi.org/10.1007/s41230-022-1105-3

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  The effect of the Mg/Si ratio of Al-2.5Li-1Cu-0.8Mg-0.8Si, Al-2.5Li-1Cu-1.6Mg-0.8Si, and Al-2.5Li-1Cu-2.4Mg-0.8Si alloys on the microstructure evolution and mechanical properties was investigated. The results show that the primary phases and their morphologies in the as-cast alloys are found to vary with the Mg/Si ratio. The improvement of Mg/Si ratio of as-cast alloys promotes the formation of Mg₂Si primary phase at the expense of the AlLiSi primary phase. Moreover, a tiny amount of T_B-Al_7.5Cu₄Li phase transforms into S-Al₂CuMg phase with the increase of Mg content. In addition, the increase of Mg/Si ratio also causes the Cu-rich intergranular phase distributed along crystal boundary to Si-rich intergranular phase. After ageing treatment, the precipitation sequence as a function of Mg/Si ratio is as follows: δ/δ'+AlLiSi (Mg/Si is ～1) → δ/δ'+β'-Mg₂Si+AlLiSi (Mg/Si is ～2) → δ/δ'+β'-Mg₂Si (Mg/Si is ～3). A good combination of strength and ductility can be obtained in Al-2.5Li-1Cu-2.4Mg-0.8Si alloy after solution and ageing. The rod-like β'-Mg₂Si precipitate has a positive influence on the comprehensive mechanical properties of the alloy.